The present invention relates to structural members used in constructing vehicle frames. More specifically, the present invention relates to structural members that are generally tubular and to a method of forming such structural members. Still more specifically, the present invention relates to structural members that are fabricated using hydroforming which are generally tubular and vary significantly in circumference, gage, or cross section along their lengths.
In many instances, it is necessary to create structural members such as frames or mounting components to provide overall support to other devices. This is particularly true in the manufacture and assembly of vehicles such as automobiles, trucks, sport utility vehicles and the like. Such a vehicle frame is shown in U.S. Pat. No. 5,149,132 entitled "Split Rear Truck Frame" which is assigned to the assignee of the present invention and is incorporated herein by reference. Another example of such a truck frame and its related mounting structures can be found in U.S. Pat. No. 5,308,115 entitled "Vehicle Frame With Overlapped Sections", also assigned to the assignee of the present invention and incorporated herein by reference.
A vehicle is assembled, at least in part, by constructing a frame and attaching components to the frame. Vehicle components may include the engine cradle, the suspension system, body panels, control arms, rear box load, cab, brake and fluid lines, and the like. The frame typically includes two generally parallel, spaced-apart side rails which run substantially the length of the vehicle. Cross-members span the distance between the side rails. Vehicle components are attached to the frame directly such as by bolting, riveting, or welding, or indirectly through brackets or other mounting structure.
Typically, components of these frames and structural members are manufactured by stamping plate steel onto desired configurations. These stamping or manufacturing operations require the use of very large presses which impart large amounts of force to a work piece. In the stamping operation, plate steel is first cut or formed into blanks of a predetermined configuration. The blanks are then placed within a press and are stamped or formed into a desired shape. For example, long pieces or blanks can be stamped into a C-shaped beam or rail. This configuration is then capable of providing greater strength when supporting or handling loads.
While stamping operations can produce components and parts in an economical fashion, several drawbacks exist. Most significantly, when stamping occurs, repeatability and consistency among parts is not always achieved. When metal is pressed into a desired shape, it tends to have an elastic characteristic causing the part to "spring back" somewhat. This spring-back characteristic is difficult to predict and is not necessarily repeatable. Consequently, high repeatability of stamped components is difficult.
Stamping operations also create inconsistencies in the work hardening of parts. More specifically, the part is "hardened" at the bend points, whereas the remaining portions of the part are generally unaffected. This results in inconsistencies in material characteristics throughout the part which can complicate the predictability of the performance of the part.
The configuration of parts is somewhat limited by stamping and bending operations. Complex parts having complicated geometries cannot always be fabricated due to limitations in the stamping process. Even when it is possible to fabricate a complex part, many separate stamping and bending operations are required to achieve the desired configuration, thus increasing costs.
A number of the parts of the frame or its components are preferably formed by generally tubular members. Tubular members are advantageous because they provide strength without excessive weight and cost and because they can easily accommodate attachment to other parts. To create tubular members and other complex geometries in a part using a stamping process, numerous individual portions of the part are typically stamped and then welded together. However, this welding process is far from ideal. Welding of numerous components requires the use of several holding or welding fixtures to configure the parts appropriately. Further, during the actual welding process, distortion is created due to heating and cooling of the parts. This distortion is very hard to control and is not necessarily repeatable, thus creating inconsistencies between components.
Mass production of stamped parts also tends to be expensive. Multiple tools are required to manufacture multiple parts. Each of these tools must be consistently designed and manufactured. The use of multiple tools complicates the manufacturing process and adds costs to the product. An additional process sometimes used for fabricating structural components is hydroforming. In the hydroforming process, a unformed part or tube is placed in a die. The interior of the tube is then pressurized causing the tube to expand to meet the interior surface of the die. By carefully configuring the die to meet the part configuration desired, tubular parts can thus be manufactured.
As is well known, the hydroforming is somewhat limited. Specifically, wide variations in cross section are required for the finished part. Hydroforming does not provide a feasible method for manufacturing. These variations require expansion of the unformed tube at a rate or level that is typically beyond acceptable levels. Therefore, this process is not easily utilized to fabricate such parts.